WriteWatermark.cs

private static void WriteWatermark(string watermarkContent, Stream originalImage, Stream newImage)
{
    originalImage.Position = 0;

    using (Image inputImage = Image.FromStream(originalImage, true))
    using (Graphics graphic = Graphics.FromImage(inputImage))
    {
        Font font = new Font("Georgia", 12, FontStyle.Bold);
        SizeF textSize = graphic.MeasureString(watermarkContent, font);

        float xCenterOfImg = (inputImage.Width / 2);
        float yPosFromBottom = (int)(inputImage.Height * 0.90) - (textSize.Height / 2);

        graphic.SmoothingMode = SmoothingMode.HighQuality;
        graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
        graphic.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAliasGridFit;

        StringFormat StrFormat = new StringFormat
        {
            Alignment = StringAlignment.Center
        };

        SolidBrush semiTransBrush2 = new SolidBrush(Color.FromArgb(153, 0, 0, 0));
        graphic.DrawString(watermarkContent, font, semiTransBrush2, xCenterOfImg + 1, yPosFromBottom + 1, StrFormat);

        SolidBrush semiTransBrush = new SolidBrush(Color.FromArgb(153, 255, 255, 255));
        graphic.DrawString(watermarkContent, font, semiTransBrush, xCenterOfImg, yPosFromBottom, StrFormat);

        graphic.Flush();
        inputImage.Save(newImage, ImageFormat.Jpeg);
    }
}